1. Field of the Invention
The present invention relates to a method and a device for detecting the tank fuel level of a motor vehicle.
2. Description of Related Art
Usually, for detecting a tank fuel level, a tank level sensor makes available a voltage signal as a function of the filling level, which is then evaluated by an instrument cluster, so that a driver may then be provided with a tank level display. Moreover, it is usual that a refueling process is also recorded with the aid of a tank fuel level sensor, by having an increase in the tank level by more than a predetermined boundary value evaluated as a sign of refueling. The tank level sensor is usually based on a potentiometer device, having a lever arm and a float gauge, in this context.
While this arrangement is technically simple and cost-effective to implement, it has the disadvantage that the lever usually has an upper dead-center position, as shown in FIG. 1, above which the tank level sensor signal UTSG—max does not change any more, even in the case of a rising tank level, and a lower dead-center position, below which the tank level sensor signal UTSG—min does not change any more, even in the case of a falling tank level. Consequently, there is an upper and a lower dead volume that cannot be detected, since the mechanical lever is at an end stop before this volume is reached.
Furthermore, deposits due to corrosion on the potentiometer path impair the accuracy and the reliability of such a tank level sensor. The corrosion problem intensifies without additional constructive measures, particularly when ethanol-based fuels are used.
A tank level sensor signal that is impaired in its reliability is not only uncomfortable for the driver, who is thus not able to estimate the cruising range to the next nearest filling station, but also leads to problems in the case of so-called FlexFuel systems, in which fuels having a different ethanol content are able to be used, and the control of the mixture is calculated as a function of the fuel used. For the calculation and determination of the ethanol content in the fuel used, detection of refueling using tank level sensor signals is used, among other things, in order to avoid unnecessary determination effort and calculation effort. Thus, a malfunction of the tank level sensor in such a FlexFuel system is able to impair the entire engine control.
Besides, the combustion of deviant fuels (such as E85 instead of gasoline) is able to be detected by monitoring changes in the engine response, such a fuel-type detection being admitted only after the detection of a refueling signal, for distinguishing actual fuel-conditioned changes in the engine response from sensor errors. In this connection, methods for detecting the fuel type are also conceivable which go back directly to the tank level signal or its change.
If an accurate and reliably detected tank level signal could be provided, it would be conceivable that exhaust gas-relevant pilot control functions and control functions would be supported by the tank level signal, and would thus use an additional, reliably detectable input parameter. This would further reduce the susceptibility, particularly of the exhaust gas-relevant control and regulating functions with respect to malfunctions of individual sensors or components.